Absorption of Drugs from the Gastrointestinal Tract

Abstract

Drugs are most often administered to man by the oral route. Compared to other modes of administration, this route is unusually difficult to describe with regard to the physico-chemical conditions existing at the site of absorption. For example, the contents of the gastrointestinal tract vary widely in pH value, volume, viscosity and composition; moreover, the rate of gastric emptying and the degree of intestinal motility are highly variable. Thus a drug is retained in the acidic environment of the stomach for a few minutes or for hours; it is propelled through the duodenal, jejunal and ileal portions of the intestine at varying rates; and it is exposed to innumerable foodstuffs, products of digestion, bacteria and digestive enzymes. As if these variables did not make the oral route difficult enough to characterize, drugs are given orally in a wide array of dosage forms, each with its own set of physical properties: there are tablets, capsules, suspensions, emulsions, a variety of solutions, and specially formulated dosage forms for prolonged or delayed action. It is not surprising then that investigators interested in the mechanisms of gastrointestinal absorption have largely turned to surgically prepared laboratory animals in which one portion of the gastrointestinal tract can be studied at a time, the nature of the luminal contents controlled, and drugs maintained in true aqueous solution.